Metal tantalum, being a valve metal, is able to form a dense oxidative film on its surface, so that the metal has the unilateral conduction property. Anode films prepared from the tantalum powder have stable chemical properties (particularly, the films are stable in an acidic electrolyte), a high electrical resistivity (7.5×1010 Ω·cm), a high dielectric constant (27.6), a small leakage current. In addition, the anode films further have the advantages of a broad working temperature range (from −80° C. to 200° C.), a high reliability, shock resistance, and a long life. Thus, tantalum powder is an ideal material for preparing tantalum capacitors having a small volume and a high reliability.
The process to prepare tantalum powder by reducing potassium fluorotantalate with sodium is a process for producing tantalum powder which is widely used all over the world and whose techniques are developed well.
The process of reducing potassium fluorotantalate with sodium is a process for making capacitor-grade tantalum powder in which K2TaF7 and Na are used as main raw materials, and NaCl, KCl and other halogen salts or a mixture of halogen salts are used as a diluent, and the main reaction mechanism of the process is shown as follows:K2TaF7+5Na=Ta+5NaF+2KF  (1)
Under the argon protection and at a certain temperature, K2TaF7 and liquid sodium take the above reaction. After the resultant tantalum powder from the reduction is subjected to water washing and acid washing, the washed tantalum powder is subjected to thermal treatments, and then, after the deoxygenation (i.e., oxygen-lowering) reduction with magnesium, highly pure tantalum powder is finally obtained. Usually, tantalum powder particles obtained by the process are in the form of granular.
As well known, the specific capacitance of tantalum powder is proportional to the specific surface area thereof, that is, a lower average particle size of tantalum powder will lead to a higher specific surface area and a higher specific capacitance. The focus of current studies is on the controls of the formation, distribution and growth of crystal nucleus during the sodium reduction by controlling the reductive conditions, including components of dissolved salt of potassium fluorotantalate and diluent, reduction temperature, sodium-injection speed and so on, so as to prepare desirable tantalum powder having a specific surface area and a particle size. A mechanical process is to obtain tantalum powder having good pressure resistance by controlling conditions of hydrogenation pulverization or milling.
Generally, the flaking of tantalum powder and the simplification of particle shape of tantalum powder are novel means for increasing the pressure resistance of tantalum powder product, and at the same time, the volume reduction caused by the means should be attended to.
The patent U.S. Pat. No. 4,740,238 is concerned to an ingot-derived unagglomerated tantalum powder composition having an average Fisher sub-sieve size (FSSS) of less than two micrometers, preferably in the range of 0.6 to 1.1 micrometers, a Scott density not greater than 30 g/in3, and a BET surface area of at least 0.7 m2/g. In one embodiment of the invention, the powder is agglomerated and milled to form a tantalum powder product having a particle size of about 2 to 3 micrometers, a BET surface area of 0.43 m2/g, and a Scott density in the range of 18 to 27 g/in3.
The U.S. Pat. No. 4,940,490 relates to an improved flaked tantalum powder and process for making the flaked powder. The powder is characterized by having a Scott density greater than 18 g/in3 and preferably at least about 90% of the flake particles being no greater than 55 micrometers. Agglomerates of the flaked tantalum powder provide improved flowability, green strength and pressing characteristics compared to conventional flaked tantalum powders.
The patent application WO93/03191 mentions an improved flaked tantalum powder and process for making the flaked powder. The powder is characterized by having a mean particle size (FSSS) in the range of 2 to 55 micrometers and a BET surface area in the range of 0.5 to 5.0 m2/g. The mean particle size and BET surface area are selected so that the flakes have an aspect ratio (D/T) in the range of 2 to 50. Agglomerates of the flaked tantalum powder provide improved flowability.
The patent U.S. Pat. No. 5,211,741 mentions an improved flake tantalum powder and a method for making a fractured flake tantalum powder. The tantalum powder is characterized by having a Scott density greater than 18 g/in3 and preferably at least 90% of the flake particles being no greater than 55 micrometers.
Agglomerates of the flaked tantalum powder provide improved flowability, green strength and pressing characteristics compared to conventional flaked tantalum powders.
The U.S. Pat. No. 5,261,942 also mentions an improved flake tantalum powder and a method for making a flake tantalum powder. The tantalum powder is characterized by having a Scott density greater than 18 g/in3 and preferably at least 90% of the flake particles being no greater than 55 micrometers. Agglomerates of the flaked tantalum powder provide improved flowability, green strength and pressing characteristics compared to conventional flaked tantalum powders. The patent U.S. Pat. No. 5,580,367 also is similar to the patent U.S. Pat. No. 4,940,490.
The patent U.S. Pat. No. 4,441,927 mentions an ingot-derived agglomerated tantalum powder composition comprising a select granular tantalum powder and including a critical proportion of a flaked tantalum powder.
The patent CN101491834A relates to a tantalum powder, a preparation method thereof and an electrolytic capacitor anode made of the tantalum powder, in particular to tantalum powder of which the BET specific surface area is not more than 0.530 m2/g and the Fisher mean particle diameter is not less than 3.00 nm. The invention also relates to the method for preparing the tantalum powder, wherein the tantalum powder is prepared by using a reducing agent to reduce a tantalum compound. The method is characterized in that tantalum powder which is taken as a crystal seed is added during reduction, which is a ground tantalum powder.
The patent WO2004110684A2 describes methods of forming tantalum powders and other valve metal powders are described. The method involves high impact milling a starting powder in a fluid medium and optionally a milling media using a high energy mill. The methods of the present invention have the ability to reduce DC leakage and/or increase capacitance capabilities of valve metal powders when formed into capacitor anodes.
The patent U.S. Pat. No. 4,555,268 relates to flaked tantalum powder having improved performance. The powder contains flaked tantalum powder and granular tantalum powder. Prior to the mixing, the mixed tantalum powder are subjected to secondary agglomeration at temperatures range from 1250° C. to 1550° C. for a period ranging from 5 to 120 minutes, so as to improve the workability.
There are many patents relating to the preparation of flaked tantalum powder from tantalum powder, however, they focus substantially on the pure flake tantalum powder. Furthermore, these processes are primarily directed to tantalum powder having a low specific capacitance of less than 30,000 μFV/g.